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Date: Tue, 12 Jul 2016 22:22:05 +0000 From: "Kani, Toshimitsu" <toshi.kani@....com> To: "snitzer@...hat.com" <snitzer@...hat.com> CC: "linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "dan.j.williams@...el.com" <dan.j.williams@...el.com>, "dm-devel@...hat.com" <dm-devel@...hat.com>, "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>, "linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>, "agk@...hat.com" <agk@...hat.com> Subject: Re: dm stripe: add DAX support On Fri, 2016-06-24 at 14:29 -0400, Mike Snitzer wrote: > > BTW, if in your testing you could evaluate/quantify any extra overhead > from DM that'd be useful to share. It could be there are bottlenecks > that need to be fixed, etc. Here are some results from fio benchmark. The test is single-threaded and is bound to one CPU. DAX LVM IOPS NOTE --------------------------------------- Y N 790K Y Y 754K 5% overhead with LVM N N 567K N Y 457K 20% overhead with LVM DAX: Y: mount -o dax,noatime, N: mount -o noatime LVM: Y: dm-linear on pmem0 device, N: pmem0 device fio: bs=4k, size=2G, direct=1, rw=randread, numjobs=1 Among the 5% overhead with DAX/LVM, the new DM direct_access interfaces account for less than 0.5%. dm_blk_direct_access 0.28% linear_direct_access 0.17% The average latency increases slightly from 0.93us to 0.95us. I think most of the overhead comes from the submit_bio() path, which is used only for accessing metadata with DAX. I believe this is due to cloning bio for each request in DM. There is 12% more L2 miss in total. Without DAX, 20% overhead is observed with LVM. Average latency increases from 1.39us to 1.82us. Without DAX, bio is cloned for both data and metadata. Thanks, -Toshi
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